1. The Field of the Invention
The present invention relates to a gas sensor, such as an oxygen sensor, an A/F (air/fuel ratio) sensor, a NOx sensor, or a HC sensor for use, for example, in controlling exhaust gas from an internal combustion engine of an automobile. More particularly, the invention relates to a prismatic multilayer ceramic gas sensor element having a substantially rectangular cross section and comprising a zirconia solid electrolyte layer and an alumina layer. The invention further relates to a method of manufacturing the gas sensor.
2. Description of the Related Art
Japanese Patent 1219 662 discloses an oxygen detection element having a rectangular cross section, comprising a laminate of a solid electrolyte plate of ZrO2, electrodes, an electrode lead formed on the solid electrolyte plate, a protection layer coated on the solid electrolyte layer and the lead, and a U-shaped frame body. The lateral surfaces of the laminate are covered with a gas-impermeable layer of alumina so as to maintain insulation resistance between the electrode and a metallic housing body even if a conductive material accumulates thereon.
United States Patent Application Publication US 2003/0159928A1 discloses a prismatic multilayer ceramic gas sensor element having a rectangular cross section, comprising a solid electrolyte layer co-fired with a ceramic beater substrate. A porous alumina layer is coated around a gas sensing cell portion formed at a distal end of the prismatic multilayered gas sensor element so as to prevent the gas sensor element from cracking induced by contact with water droplets.
3. Problems to be Solved by the Invention
The present inventors have found that carbon or soot exhausted from an internal combustion engine, and accumulating on the lateral surface of a prismatic multilayer ceramic gas sensor element, may cause “blackening” or deoxidization of the solid electrolyte layer laminated in a posterior lead portion adjoining a gas sensing cell portion of the gas sensor element.
The deoxidized solid electrolyte layer may cause serious problems, including reduced insulation resistance between the metallic lead connected to a gas sensing cell electrode formed at a distal end portion of the prismatic multilayer gas sensor element and a metallic housing supporting the gas sensor element at its posterior lead portion adjoining the gas sensing cell portion, a malfunctioning gas sensing mechanism, and weakened mechanical strength due to brittleness of the deoxidized solid electrolyte layer.
Conventionally, a prismatic multilayer gas sensor element has been made by cutting a sheet-like laminate comprised of an oxygen-ion conductive solid electrolyte layer and insulating ceramic layers sandwiching the solid electrolyte layer, and coating cut-surfaces (corresponding to the lateral surfaces of the prismatic multilayer ceramic gas sensor element) with an insulating protective layer so as to prevent the oxygen ion conductive solid electrolyte layer from being directly exposed to an ambient gas atmosphere.
An analysis conducted by the present inventors indicated that this “blackening” or deoxidization of the solid electrolyte layer starts to occur particularly at the cut-surfaces corresponding to the lateral surfaces of the posterior lead portion adjoining the gas sensing portion. In this regard, carbon-like soot accumulates on the protective layer formed on the lateral surfaces when an electrical voltage is applied across the leads sandwiching the solid electrolyte layer and a metallic housing supporting the posterior lead portion of the prismatic gas sensor.
Notably, the deoxidization of the zirconia solid electrolyte layer does not occur at the gas sensing cell portion of the prismatic multilayer gas sensor, while it may occur at the posterior lead portion adjoining the gas sensing cell portion. The reason for this is considered to be as follows. Since the posterior lead portion in which a zirconia solid electrolyte layer is sandwiched by metallic leads running from the gas sensing cell electrodes formed at a distal end portion of the prismatic sensor element to a rear end potion thereof is supported by a housing member for holding the gas sensor element, the temperature of the posterior lead portion can not be elevated to burn off carbon-like soot accumulating thereon. On the other hand, the gas sensing cell portion is heated to a temperature that is high enough to burn off the carbon-like soot.
Therefore, there is a need for an improved protective insulating layer for coating on the lateral surfaces of the lead portion of a prismatic gas sensor in view of resistance against carbon or soot-fouling, as well as an improved thermo-positional relationship between the gas sensing cell portion and the posterior lead portion in view of effective thermal oxidation or burning off of carbon-like soot accumulating on the prismatic multilayer ceramic gas sensor element.